In-field use of the earliest mechanical tomato harvesters pointed to the need for a more resilient tomato. While greatly reducing labor and time requirements associated with harvesting, the mechanical harvesters caused considerable damage to the crop itself. The early harvesters' rough handling of the tomatoes was improved somewhat by later designs, but the greater reduction in crop losses resulted from the development of new strains of tomatoes with hardy physical characteristics.
While the new tomatoes proved able to withstand the harvesting experience, they also were a much more difficult tomato to separate from the vine than earlier varieties. Tomato vines passed through the harvester and fell back to the ground with substantial numbers of tomatoes still attached.
Conventional tomato harvesters employ a shaker which accepts the severed tomato vines and subjects them to an oscillating forward and reverse action. Prior art shakers have been driven by a number of different mechanical drive units that effect the desired forward and reverse motion. For example, R. L. Button, U.S. Pat. No. 3,364,770 discloses a periodically varying drive mechanism, particularly suitable for use with a shaker conveyor. Similarly, Johnson, U.S. Pat. No. 3,721,132 illustrates an intermittent drive for shaker conveyer, such as might be used in a tomato harvester. Both devices exhibit a certain forward/reverse-acceleration/deceleration action which, if graphically depicted, approximates a sine-wave function. The sinusoidal action of both drive mechanisms stems from the use of a member tangentially interconnected to a rotating disc or wheel. This cycle-producing combination determines the overall characteristics of the shaker conveyer's reciprocating action.
The present invention relies on electronic rather than mechanical means to produce its control cycle. The electronic signal produced by the control circuitry exhibits a substantially square-wave characteristic in lieu of the sinusoidal cycle described above. In contrast to the relatively slow rate of change of the sine wave from 0.degree. to 90.degree., and every quarter cycle thereafter, the square-wave displays a near instantaneous rate of change through each quarter-cycle.
The pulsating square-wave controls a solenoid-operated hydraulic valve. A pair of hydraulic motors is interconnected to a shaker conveyer such as a plurality of shaker chains, or the like. In an alternating fashion, one motor drives the conveyer forwardly and then the other drives the conveyer rearwardly at the command of the hydraulic valve. While the resultant mechanical motion cannot duplicate the instantaneous electronic control pulse, it resembles the square-wave function and is quite different from the sinusoidal shaking operation of prior art mechanisms known to applicants.
The quick, snapping action of a shaker bed driven by the present invention proves much more effective in separating comestibles from their supportive plant or vine than the comparatively restrained operation of prior art devices. Even the new generation of plants developed for machine harvesting can effectively be stripped of its fruit when subjected to the whip-like action of such rapid forward and reverse motion. Features which permit shaker frequency to be continuously variable assure that a frequency appropriate to the particular variety of comestible to be harvested can be chosen by the operator. Thus, losses which might result from over agitation of the crop can be easily avoided.